Only after he realized the significance of mastering two languages English carefully.
A. did he set about learning
B. did he set out learning
C. had he set about learning
D. had he set out learning
网友回答
(答案→)A. did he set about learning
B. did he set out learning
C. had he set about learning
D. had he set out learning
解析：考查倒装句,当only谓语句首时,主句部分采用部分倒装.句意,只有他认识到掌握两种语言的重要性后,他才开始认真学英语.set about doing sth开始做某事; set out出发;开始,后接不定式.故选A.